To realize a data rate over 100 Mbps for next generation mobile communication systems, various wireless communication methods adequate to high-speed packet transmission are considered. Frequency bands to be used need to be wider for realizing that high-speed transmission and around 100 MHz band is considered for use.
It is known that, if such wide band transmission is performed in mobile communication, a communication channel is a frequency selective channel comprised of a plurality of paths with respective delay times. Therefore, in wide band transmission in mobile communication, intersymbol interference (hereinafter “ISI”) where a symbol interferes to the following symbol is caused and bit error rate (hereinafter “BER”) performances are degraded. Further, in a frequency selective channel, the channel transfer function varies in the frequency band, resulting in spectrum distortion of signals received via this channel.
There is an equalization technique for removing the influence of ISI and improving the BER performances. To be more specific, maximum likelihood sequence estimation (hereinafter “MLSE”) is known as a time domain equalization technique. However, in MLSE, the structure of an equalizer becomes complicated with an increase of the number of paths significantly, and the amount of calculation for equalization increases exponentially. Consequently, as an equalization technique where the structure of an equalizer does not depend on the number of paths, frequency domain equalization (hereinafter “FDE”) has attracted attention recently (for example, see Non-Patent Document 1).
In FDE, a received signal block is divided into orthogonal frequency components by the fast Fourier transform (hereinafter “FFT”), and these frequency components are multiplied with equalization weights approximate to the reciprocal of the channel transfer function, and converted into time domain signals by the inverse fast Fourier transform (hereinafter “IFFT”). By this FDE, it is possible to compensate spectrum distortion of received signals, and, as a result, ISI is decreased and the BER performances are improved. Further, in equalization weights, minimum mean square error (hereinafter “MMSE”) weight, which minimizes the mean square error of the frequency component and the transmission signal component subjected to equalization, provides the best BER performances.
In the technique disclosed in above-described Non-Patent document 1, a received signal needs to be provided as a repeat signal with the FFT block length, and, consequently, the same signal as the tail part of a symbol is attached to the head of the symbol block as cyclic prefix (hereinafter “CP”) on the transmitting side. Further, by providing CP, it is possible to prevent ISI as long as the delay time of delay waves stays within the time length of CP (hereinafter “CP length”). Here, CP may be referred to as a guard interval (hereinafter “GI”).
Non-Patent Document 1: D. Falconer, S.L. Ariyavistakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems”, IEEE Commun. Mag., vol. 40, pp. 58-66, April 2002